This invention relates to a method of feeding wastes into a boiler, more particularly, to a method of heat treatment by which municipal wastes, night soil, sewage sludge, industrial wastes and other forms of wastes that contain organic matter, are converted into a fuel in the presence of water such that it can be fed into a boiler, as well as a method by which high-moisture wastes containing halogens can be converted into a fuel for supply into a boiler.
The following is one method in the R&D stage for feeding wastes into a boiler. A solids-containing waste is finely divided and converted into a slurry, from which inorganics such as glass, tiles and pebbles, metals, etc. are removed as much as possible. If the waste is already of a fine size and need not be ground or if it is substantially free of solids as in the case of sludge or liquid waste, it may optionally be passed through a screen to remove coarse solids. Then, the slurry is pressurized, heated to a suitable temperature between 250.degree. C. and 350.degree. C. depending upon the type of waste and held at that temperature typically for several tens of minutes in order to cause a hydrothermal reaction. As a result, the organic matter in the waste is dehalogenated and/or deoxygenated in the form of CO.sub.2 such that it is converted to carbides (generally called "char"), pitch or tar-like oil, from which an aqueous phase is removed to form a fuel.
In this method, the slurry to be pressurized is mixed with a sufficient amount of alkali to ensure that the slurry remains alkaline and will not become acidic even if it is subjected to a hydrothermal reaction. This alkali addition is performed in order to increase the yield of oil as a reaction product, and examples of the alkali commonly added include Na.sub.2 CO.sub.3, NaOH and Ca(OH).sub.2. If hydrochloric acid or other strong acids are formed during the reaction, the pH of the slurry decreases so much that those parts of the reactor, e.g. heat exchangers, pipes and other system components, which are in contact with the liquid will be attacked by severe corrosion and alkalies must be added in order to prevent this problem.
Such method, however, has had the following limitations. If a hydrothermal reaction is performed in the presence of an alkali, there will be extensive production of alcohols, ketones, aldehydes, organic acids and other reaction products that are not easily separable from water. This means that as the result of treatment by the hydrothermal reaction, a substantial portion of high heat value of the feed waste is transferred into the aqueous phase and by subsequent removal of the aqueous phase from the reaction products, such portion of high heat value is lost, resulting in a lower fuel yield.
In addition, the effluent resulting from the removal of the aqueous phase has very high BOD and COD levels (&gt;several tens of thousand ppm) and cannot be treated easily.
Further in addition, if the waste contains alkaline earth metal elements such as calcium and magnesium, the latter will enter into a precipitation reaction with sulfate and carbonates that have formed in consequence of the sulfur oxides and carbonic acid that have occurred in the hydrothermal reaction, causing scale deposition on the surfaces of those areas of the reactor, heat exchangers, pipes and other system components which are in contact with the liquid; the scale will gradually grow until operation of the system is no longer possible.